Physical Solubility and Diffusivity of CO 2 in Aqueous Diethanolamine Solutions Mousa K. Abu-Arabi,* Asem M. Al-Jarrah, Mohammed El-Eideh, and A. Tamimi Department of Chemical Engineering, Jordan University of Science & Technology, Irbid, Jordan The physical solubility and diffusivity of CO 2 in aqueous DEA solutions were measured directly in this work via the “protonation method”. The concentration of DEA in the aqueous solutions was 10, 20, and 30 mass %. The measurements were done in the temperature range (25-60) °C at atmospheric pressure. Correlations for the solubility and diffusivity as a function of temperature were developed. The results were compared with the values calculated on the basis of the N 2 O analogy. New correlations for the solubilities and diffusivities of CO 2 and N 2 O in water were also developed. 1. Introduction Aqueous alkanolamine solutions are commonly used in gas-treating processes to remove acid gases, like H 2 S and CO 2 , from natural and industrial gases. Monoethanolamine (MEA), diethanolamine (DEA), di-2-propoanolamine (DIPA), and N-methyldiethanolamine (MDEA) are some of the amines widely used for this purpose. In the design of a gas- treating process for the removal of these gases, it is essential to calculate the mass-transfer rates accurately. Apart from information needed on the mass transfer, gas- liquid contact area, and reaction kinetics, data on the fundamental physicochemical properties such as physical solubility and diffusivity are needed. The physical solubility is the free-gas solubility that dissolves in the solvent. The reactivity of acid gases with alkanolamine solutions makes the direct measurements of the physicochemical properties impossible. This led Clarke, 1 and later many researchers in this field, 2-12 to estimate these properties from corresponding data of similar nonreacting gases. For CO 2 , the analogy with N 2 O, because of molecular similari- ties, is widely applied and has been referred to as the “N 2 O analogy”. Clarke assumed that the ratios of the solubilities and diffusivities of CO 2 and N 2 O in water and in aqueous solutions of organic solvents are similar within 5% at the same temperature. The relations that have been used to calculate the solubility and diffusivity of CO 2 in amine solutions on the basis of the N 2 O analogy are The solubility is expressed in terms of H, the Henry’s law constant. H and D of CO 2 and N 2 O in water and H and D of N 2 O in the amine are calculated at the particular temperature at which the solubility or the diffusivity of CO 2 in the amine is required to be estimated. To calculate the physical solubility of CO 2 in amines via the N 2 O analogy requires three measurements; the physical solubility of CO 2 and N 2 O in water, and the solubility of N 2 O in the amine. For the diffusivity, six measurements are required; the physical solubility and the diffusivity of CO 2 and N 2 O in water and the physical solubility and diffusivity of N 2 O in the amine. Beside the efforts and cost to conduct the required measurements, in each measurement there is usually a certain experimental error, which can accumulate to give a high percentage of error in the calculated value. Abu-Arabi et al. 13 developed a “protonation method” to measure directly the physical solubility and diffusivity of acid gases in alkanolamine solutions. The method is based on the protonation of the solutions with HCl before * To whom correspondence should be addressed. On leave at The Middle East Desalination Research Center, P. O. Box 21, Al-Khuwair PC 133, Muscat, Oman. E-mail: mousa@medrc.org.om. Fax: + 968 697 107. Table 1. Solubility of CO2 in Water H/(Pa‚m 3 ‚mol -1 ) t/°C ref 5 ref 18 ref 19 ref 10 ref 12 this work 15 2240.3 18 2469.1 19 2490.2 20 2631.6 2589.9 25 3003.5 2994 2984 2993 2980.1 30 3571.4 3394.4 3382 35 3937.1 3809.8 37 3897.1 38.4 4098.4 40 4219.4 4249.6 4227 40.4 4201.7 45 4854.4 4689.3 50 5154.6 5166.6 5136 56 5823.3 60 6134.9 70.5 7142.9 77.2 7575.8 82.2 8333.3 87.1 9259.3 Figure 1. Solubility of CO2 in water: O, this work; 2, Versteeg and van Swaaij; 5 ×, Saha et al.; 10 /, Li and Lai; 12 9, Yasunishi and Yoshida; 18 [, Int. Crit. Tables; 19 s, fitted line. (H CO 2 ) amine ) (H CO 2 /H N 2 O ) water (H N 2 O ) amine (1) (D CO 2 ) amine ) (D CO 2 /D N 2 O ) water (D N 2 O ) amine (2) 516 J. Chem. Eng. Data 2001, 46, 516-521 10.1021/je000222c CCC: $20.00 © 2001 American Chemical Society Published on Web 02/22/2001